1. Field of the Invention
The present invention relates to current measuring circuits, and more particularly to a current measuring circuit for measuring a current driven to a load by a direct-current (DC) or alternating-current (AC) power supply.
2. Description of the Background Art
In a current measuring circuit for use in a current measuring test of an integrated circuit (IC) to be measured, various methods for high-precision measurement of a current passing through a load configured with the IC to be measured have conventionally been proposed, as described in Japanese Patent Laying-Open No. 11-23664, for example.
FIG. 18 shows a configuration of a conventional current measuring circuit proposed in Japanese Patent Laying-Open No. 11-23664.
Referring to FIG. 18, the conventional current measuring circuit includes a load 1, a voltage apply terminal 2 for applying a voltage to load 1, a voltage supply terminal 3 for supplying a first voltage V1 determining the applied voltage to load 1, an operational amplifier 4, NPN transistors Q9, Q10, resistances R1, R2, and a voltage supply current measuring terminal 5 measuring with an amperemeter 6 a current flowing when a second voltage V2 is supplied.
Load 1 is connected between voltage apply terminal 2 and a ground potential, and a current in accordance with the applied voltage of voltage apply terminal 2 is driven to load 1.
NPN transistors Q9, Q10 constitute a current mirror circuit, with a base of NPN transistor Q9 being connected to a collector of NPN transistor Q9 and a base of NPN transistor Q10. The base and the collector of NPN transistor Q9 and the base of NPN transistor Q10 are further connected to an output terminal of operational amplifier 4. A collector of NPN transistor Q10 is connected to an external power supply terminal VCC.
Resistance R1 has one end connected to an emitter of NPN transistor Q9 and the other end connected to an inverted input terminal of operational amplifier 4 and to voltage apply terminal 2 of load 1.
Resistance R2 has one end connected to an emitter of NPN transistor Q10 and the other end connected to voltage supply current measuring terminal 5. Voltage supply current measuring terminal 5 consists of amperemeter 6, and a voltage source 7 connected between amperemeter 6 and a ground potential to supply voltage V2.
In the configuration described above, when first voltage V1 supplied to voltage supply terminal 3 is input to a non-inverted input terminal of operational amplifier 4, the input voltage of the inverted input terminal can be considered to be equal to voltage V1, since operational amplifier 4 whose differential gain is sufficiently large has a difference input of approximately zero. Accordingly, voltage apply terminal 2 of load 1 connected to the inverted input terminal is applied with a voltage equal to voltage V1.
Further, a current flowing through load 1 corresponds to a current I9 which flows from the output terminal of operational amplifier 4 via NPN transistor Q9 and resistance R1 into voltage apply terminal 2. Since NPN transistors Q9 and Q10 constitute a current mirror circuit, a mirror current I10 of current I9 flowing through NPN transistor Q9 flows through NPN transistor Q10 and resistance R2.
Here, if voltage V2 of voltage source 7 is set equal to first voltage V1 at voltage supply current measuring terminal 5, current I9 of load 1 and mirror current I10 will be proportional to a ratio between reciprocals of resistances R1 and R2, which can be represented as I10=(R1/R2)·I9. Thus, current I9 flowing through load 1 can be obtained by measuring mirror current I10 with amperemeter 6 inside voltage supply current measuring terminal 5.
As such, in the conventional current measuring circuit shown in FIG. 18, a current mirror configuration is employed, and a terminal for applying a voltage to load 1 and a terminal for measuring a current flowing through load 1 are provided separately. This ensures uninterrupted voltage supply even if a current measurement range is switched, and thus, it is possible to measure currents of various levels flowing through voltage apply terminal 2 with proper current measurement ranges, permitting high-precision measurement.
With the conventional current measuring circuit, however, current I9 of load 1 is measured on the assumption that applied voltage V1 of load 1 is equal to voltage V2 of voltage source 7 inside voltage supply current measuring terminal 5. This would cause a need to change voltage V2 of voltage source 7 when applied voltage V1 of load 1 is changed, thereby complicating the measurement.
In addition, assume that applied voltage V1 of load 1 is an alternating voltage, or load 1 is capacitive. In such a case, when a ground voltage terminal VEE of operational amplifier 4 is changed to a negative source, although it may be possible to measure a current flowing from voltage apply terminal 2 into load 1 (hereinafter, referred to as the “source current”), it would not be able to measure a current flowing out of load 1 (hereinafter, referred to as the “sink current”), since NPN transistors Q9 and Q10 turn off and cannot pass currents therethrough.